The invention relates to a relative pressure measuring instrument. A relative pressure measuring instrument is used to detect a pressure p to be measured with reference to a reference pressure pR. The pressure thus measured is denoted as relative pressure. As an example, an atmospheric pressure that prevails in an environment of a measuring point serves as reference pressure, for example.
Relative pressure measuring instruments are used in virtually all branches of industry for measuring relative pressures. The measured relative pressure values are used, for example, to control, regulate and/or monitor an industrial production process and/or processing procedure.
Relative pressure measuring instruments have a relative pressure measuring cell installed in a housing. The relative pressure measuring cell has, for example, a basic body and, arranged thereon, a pressure sensitive element, for example a diaphragm. The pressure p to be measured acts on a side of the pressure sensitive element averted from the basic body either directly or in a fashion transmitted by a pressure transmitter. The reference pressure pR acts on a side of the pressure sensitive element facing the basic body. The reference pressure pR is fed to the pressure sensitive element via a reference pressure feed, as a rule a bore that penetrates the basic body and opens in the housing.
The relative pressure measuring cell is customarily arranged on a support by which the housing is subdivided into a process-facing half and a process-averted half. During operation, the pressure p to be measured is fed through the process-facing half.
Arranged in the process-averted half is an electronic circuit for receiving, evaluating and/or processing electric signals generated by the relative pressure measuring cell.
The support preferably forms a pressure-proof separation that serves the purpose of preventing a medium whose pressure is to be measured from emerging.
However, in the case of a relative pressure measuring cell, it can happen that the pressure sensitive element is destroyed. In the case of relative pressure measuring instruments in which the reference pressure feed opens in the process-averted housing half, the reference pressure feed then effects a connection between the two housing halves. In the worst case, the medium destroys an electronic system located in the second housing half before it can output a fault message.
DE-690 15 367 T2 describes a reference pressure measuring instrument that has a cylindrical support that is enclosed in a cylindrical housing. The support is welded onto a process connection that is screwed into the housing. A reference pressure feed is provided that is led laterally out of the support and opens at a side wall of the support facing the housing. An annular cylindrical cavity between housing and support, into which the reference pressure feed opens, is sealed, on the one hand, by the thread of the process connection and, on the other hand, by a seal held between the housing and the support in a groove in the support.
If the medium emerges here through the reference pressure feed, it can flow off outwards through the thread only poorly. The medium will correspondingly have intimate contact with the seal. In the case of particularly aggressive media, it may be that the seal is damaged by the medium and loses its effect. In this case, the medium can penetrate again into the process-averted housing half.
U.S. Pat. No. 6,050,145 describes a relative pressure measuring instrument that is very similar to the relative pressure measuring instrument described in DE-690 15 367 T2. The support here is welded onto a pressure transmitter that is screwed into the housing. A reference pressure feed is provided that runs a short distance in the support in the process-averted direction, is then continued laterally and is finally led out of the support past the relative pressure measuring cell in the process-facing direction. Outside the carrier, the reference pressure feed is continued in the pressure transmitter and opens below the housing in the process-facing direction. If destruction of a separating diaphragm of the pressure transmitter and of the diaphragm of the pressure measuring cell occurs here, the medium is led out from the housing.
This type of reference pressure feed is, however, very complicated, since it requires a multiplicity of bores through the support (metallic as a rule) and the pressure transmitter, in order to construct the conduit system.
It is an object of the invention to provide a relative pressure measuring instrument having a reference pressure feed that offers a high measure of safety and is simple to produce.
For this purpose, the invention comprises a relative pressure measuring instrument having
a relative pressure measuring cell
having a pressure sensitive element,
on whose process-facing side there is present during operation a pressure p to be measured,
on whose process-averted side there is present during operation a reference pressure pR to which the pressure p to be measured is to be referred,
a support
on which the relative pressure measuring cell is arranged and
which is enclosed in a housing,
a process connection,
that is connected to a process-facing end of the housing,
a measuring instrument housing
that is arranged at a process-averted end of the housing, and
a reference pressure feed through which during operation the reference pressure pR is led to the process-averted side of the pressure sensitive element,
and which leads completely out of the relative pressure measuring instrument laterally through the support and a single bore through the housing.
In accordance with one development, the support consists of glass. An electromechanical transducer is provided that serves the purpose of detecting a pressure dependent variation in the pressure sensitive element and converting it into a pressure dependent electrical variable that can be tapped via connecting lines. Contact pins connected to the connecting lines are glazed in and guided through into the process-averted half of the housing.
In accordance with one development, an adapter is provided that is connected to the housing and on which the measuring instrument housing is mounted.
In accordance with one development, the support consists of an insulator, and the pressure sensitive element is applied directly to the support.
In accordance with one refinement, there is inserted into the bore in the housing a filter that serves the purpose of preventing the intrusion of liquid and/or moisture.
In accordance with one refinement, the bore in the housing is provided with a cover that serves as splashwater guard.
In accordance with one refinement, at least one conduit section of the reference pressure feed has a smaller diameter and serves as flame barrier.